Organization of spinal cord notes

The Spinal Cord: Review
- Ends at L2 because the growth of the neuro system Is slower than the vertebral column
o Spinal nerves continue to exit  cauda equina
- Spinal cord is not uniform in length, enlargements in Cervical & Lumbosacral regions
o Enlargements are due to presence of limbs (brachial plexus coming from C5-T1,
and lumbosacral plexus from L1-S4
Slide 2:
Neuroepithelium develops a
segmental organization  develops into neurons & glial cells
o Segmental organization runs from cephalic region  coccyx
Slide 3:
The lumen of the neural tube becomes the central canal of the cord, and
ventricles of the CNS filled with CSF
o Ventricular system springs from central canal, which is filled with CNS
o Enlarged central canal
 Central canal can develop a cyst or syrinx (fluid filled cavity)
o As canal gets large, it puts pressure on axons that may be next to
Slide 7-9
- Notochord
serves as
primitive spinal
- Presence of
ectoderm to
form the neural
- This then
induces the
into somites
which give rise
to sclerotome
myotome (muscles), dermatome (skin)
Slide 10-13:
- At each vertebral level, each segment of the neural tube innervates the
dermatome/myotome of the somite associated with it
- Because somites have segmental organization, spinal cord also has segmental
- Dorsal ½ of spinal cord  sensory
- Ventral ½ of spinal cord  motor
o Innervation of muscles have a segmental organization:
 Flexing the hip: L2, L3
 Extending the knee: L3, L4
 Extending the ankle: L4, L5
- Sensory innervation of the skin is segmented into dermatomes
Slide 14: Adult Spinal Cord
- Central canal: runs length of
the spinal cord, filled with CSF,
size can enlarge and put
pressure on grey matter and
nerves associated
- Grey matter: Neural
epithelium (lots of cells)
- White matter: axons of the
neural epithelium that are
- Ventral horn  motor
- Dorsal horn  sensory
Slide 15: Thoracic Spinal Cord
- Distinguishing feature of
thoracic spinal cord 
Presence of Lateral Horn
found at T1-L2
- Reason for lateral horn: it
gives rise to preganglionic
sympathetic neurons that
provide sympathetic
innervation to the entire
Ventral White Commissure: Goes left  right,
o Associated with axons communicating b/w two sides of the spinal cord
o If central canal gets larger, pressure will have greatest detriment to the white
Anterior ventral median fissure
Posterior median sulcus (useful for distinguishing right vs. left of spinal cord)
Posterior lateral sulcus (for distinguishing white matter of spinal cord into column of
Anterior lateral sulcus  where ventral roots exit
Slide 16: Types of Neurons found in the spinal cord
- Motor neurons
o Innervating skeletal muscles or autonomic structures
o Located primarily in ventral horn
o Multipolar spinal motorneurons
- Interneurons
o They allow for elaboration of sensory signals and development of reflexes and
complex circuits like walking, speech, movement
o Functions- Important for turning on/off groups of neurons and processing post
sensory neural information
 excitatory or inhibitory
o Types:
 Intrasegmental interneurons (Renhsaw cell)
 The axons of these neurons stay within a single spinal cord area
(e.g., an axon at T1 will not do anything outside of this local T1
 Propriospinal interneurons
 Function – relaying information in a single spinal cord segment to
other segments of spinal cords
 Short (3 segments, unilateral)
o Does not cross limbs or body regions
o Axons can span 2-3 spinal cord segments
o Interconnect the motor neuron pools and connecting
reflexes of uMN and LMN to work as a unit
 Intermediate
o Helpful in coordinating the activity/movement of different
 Long (entire length of cord, bilateral)
o When we move, pair of spinal cord muscles will move as a
unit (e.g, muscles in neck with muscles in the trunk)
- Projection neurons
o Sensory information making synpases in grey matter, and then information is
relayed to higher level (brainstem, thalamus, cortex0
o Sending info from spinal cord  high level
Slide 18: (Testing for various interneurons)
Intrasegmental interneuron
Researcher dyed a
single neuron to fill up and
able to track it
Long Propriospinal interneuron
- Dissection of cats spinal cord
Dye injected, taken
up by neuron synapses and
the dye is transported back,
through retrograde
transport, back to the cell
body of the axon
Black dots on both
sides of the spinal cord,
showing interneurons that
are filled by the dye
Presence of long
axons connecting the
movement of segments of
upper & lower limb
Short propriospinal interneurons
- Gave electrical shocks
in one segment of
spinal cord and then
measured this 2-3
segments below
where they stimulated
- Spikes in A show
artificial stimulation,
causing AP
propagation in
opposite direction,
and a characteristic
spike (antidromic
- As compared to
regular synaptic
activity spike that
follows the first
Stimulation in this backfiring manner activates interneurons in that area  used to
demonstrate, which tracing axons, them going from a place of stimulation to a place of
recording and demonstrating the presence of short interneurons
Slide 21:
- Medial motor systems: control axial
musculature – spinal muscles, muscles
of the abdominal wall
- Lateral motor systems:
- In general, we see connecting the
periaxial musculature are long
propriospinal interneurons
- Short propriospinal interneurons will
connect lateral motor groups
- Axons are running in a nucleus in an
interphase along white matter, called
the fasiculus propias
Anatomy of the Gray Matter: Ventral Horn
Motor neurons that innervate a single muscle are clumped together in a single place,
form motor nuclei
o Different clumps innervate different muscles relative to other motor nuclei
o In 3D, motor nuclei actually appear in columns , in cross-section motor nuclei
appear circular or ovoid
- Motor neurons located
medially (recall medial motor
systems)  neurons
innervating the paraxial or
para spinal muscles are
located more medially
As we move more
laterally out, we find motor
neurons that are more
innervating the hand
- Flexor/extensors are not
Flexors  more
dorsally located
Extensors  more
ventrally located
Cells within the grey
matter are organized into
10 lamina
Motor neurons are
found in lamina IX (Medial
vs. lateral)
Lateral group
appendicular musculature
will be well developed
particularly where brachial
& lumbosacral plexus arise,
which can be helpful for
knowing where you are
looking at along the spine
A large lateral group
 you are likely where
limbs are
Lamina VII:
- Intermediolateral cell
column  where
motor neurons of
sympathetic cells are
found  presence of
these cells give us
lateral expansion
- Clarkes column 
part of spinal cerebella
system dealing with
Lamina VIII:
Located more medially surrounding parts of medial IX
Lots of polysynaptic reflexes
Have many motor interneurons that are involved in reflexes
Helps regulate gamma motor activity (contribute to setting sensitivity of motor spindle)
Regulates resting tone  all muscles have resting tension & conditions can occur where
tone is too much (spasticity) or too is too little (waiters tip look due to imbalance of
muscle tension)
Anatomy of the Gray Matter: Dorsal Horn
Dorsolateral zone of Lissauer:
o Axons that are going b/w different segments, when dorsal root reaches
dorsolateral zone, axons bifurcate (some axons go up, some go down), and thus
 Information from this dorsal root is affecting several spinal cord
 Sensory pain afferents can ascend & descend to influence more than aa
single segment of the spinal cord
Marginal zone & substantia gelatinosa  nocireceptor/pain information part of the
spinal cord
Nucleus proprius  information from other modalities (vibration, touch, fine touch) 
- DZ of Lissaeur has small diameter
axons that come in & go to
different spinal cord levels
- entry zone for the dorsal root
with mostly pain information
((small axons) coming into the
lateral division
- Pain fibers run through
3-4 spinal cord segments,
influence activity of
different spinal cord
- important for identifying
clinical manifestations of
pain or loss of pain due to
lesion, etc
- Type 3 fibers, or Adelta fibers
- Lamina 5  includes
sensory integration of pain,
vibration, proprioception
- also Lamina 5, you will
have projection neurons,
which will project info to
higher levels in the brain
- C fibers are
unmyelinated sensory
axons (oldest fibers,
slowest conduction of
information, associated
with visceral
information as well)
- lateral group of axons
synapsing largely at
lamina 1, 2
- axons come in and
mostly go to the dorsal
funiculus  this is info
that will ascend up the
brain stem & eventually
reach to cortex, telling
us what’s going on with
our skin in terms of fine
touch, vibration,
- type II fibers are
second largest that will
go to the middle part of
dorsal horn with a lot of
sensory information,
interacting with lamina
3, 4, 5
- largest & fastest conductors
 carrying proprioception
from muscle spindles & golgi
tendon organs
- part of lateral division of
dorsal horn, where most axons
are going straight up to the
- Axons also give off branches
(collaterals) that target Lamina
- Monosynpatic input going
from large A alpha axons to
lamina 9  a knee jerk reflex
uses such fibers (monosynaptic
- Lamina 7 & 8 are also
targeted, where we find motor interneurons that connect different motor neurons involving
synaptic reflex activity
axial section through dorsal
roots reveal afferents of different
These are sensory axons, and
we see axons with different sizes
Nerve fibers are classified
according to: diameter, degree of
myelination, and speed of
Axons entering in the dorsal root divide into two divisions: according to axons diameters
- medial fibers run
most medially 
largest fibers (Type
1, Type II)
- lateral division 
fibers that are
smaller (Type 3, 4)
Anatomy of the White Matter, Dorsal Horn
Like a nerve, white matter is composed of myelinated axons (cross section looks like a
o Myelin from Oligodendrocytes, not Schwann cells
o No CT in CNS, so not regrowth
o Fiber tracts are bundled axons that have specific function (motor or sensory)
- posterior 
dorsal roots
come off here,
located between
the two sulcus
White matter is
organized into ascending
(sensory – blue) and
descending (motor – red(
3 long tracts: lateral
spinothalamic tract, dorsal
columns, corticospinal
Funiculi are
associated with different
and specific tracts
1. Spinothalamic tract  in anterolateral funiculus  carries pain & information &
temperature & crude touch
a. One of the oldest tracts in vertebrae system
2. Dorsal column  carries information about fine touch & proprioception  fast patheay
3. Corticospinal tract  allows us to have voluntary movements
a. Lateral tract in dorsolateral funiculus gives us voluntary control of fingers, hands,
4. Anterior part of corticospinal  controls axial musculature
Note: the amount of white matter decreases at lower levels of the cord…why?
locating lateral horn presence or not (thoracic region)
Cervical has a large lateral horn  lateral motor groove sending out axons of the
muscles of the limb and giving a large expansion
o Also large amount of white matter
o Axons for ascending & descending pathways
Lumbar level
o Lateral motor groove associated with this level, but we see more white matter
bc white matter sensory tracts will be larger here
o Descending motor tracts are coming into lumbar level and may be passing
through to sacral
Sacral level
o Expansion of ventral horn due to sacral spinal cord innervating lower limbs &
need room those motor neurons
o Not much white matter
o We have: sensory & motor forming ascending and descending tracts
Blood supply to the spinal cord
- stroke
anterior spinal
artery will
bilateral &
- obstruction of
spinal artery
will lead to a
block of supply
and will impact
- vertebrae arteries run in the
transverse cervical foramen
- medullary arteries join the
anterior & posterior spinal
arteries, reinforcing blood supply
to the arteries
- Adamkiewicz is very large
typically & comes off to the left at
T10.  Intervertebral foramen at
T10 is typically much larger
- the main anastomoses providing
blood to lower part of spinal cord
& corticospinal tracts  occlusion
of artery will lead to deprived
blood from the lumbosacral
enlargements in distal part of
spinal cord  paraplegia
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